1. Field of the Invention
The invention relates to topical antibacterial ointments containing yellow mercuric oxide.
2. Description of the Prior Art
Yellow mercuric oxide, dispersed in an oleaginous ointment base, has been shown to be a safe and effective treatment for topical infections, especially near the eye. However, yellow mercuric oxide is sensitive to light, moisture and reducing agents, producing a green to black discoloration on the outer surface of the exposed yellow mercuric oxide particles. Since the discoloration occurs only at the surface, it is of little more than aesthetic importance and usually does not affect the safety or effectiveness of the yellow mercuric oxide ointment. The discoloration does have a significant effect upon user acceptance, however, and for this reason needs to be prevented.
Many of the prior art ointments containing yellow mercuric oxide also contain a small quantity of boric acid to stabilize the mercuric oxide against color/chemical deterioration. The use of boric acid and related compounds as stabilizers in antibacterial ointments are disclosed, for example, in U.S. Pat. No. 1,493,564. However, even boric acid-containing ointments generally have shelf lives of no more than about six months before serious discoloration of the mercuric oxide active ingredient makes the composition unacceptable to the ordinary consumer/user.
It has been further proposed in the prior art to utilize a combination of boric acid and wheat germ oil to prevent deterioration and discoloration of yellow mercuric oxide in an ointment base, and to potentiate the bactericidal potency of the active ingredient (see U.S. Pat. No. 4,612,193). While ointment compositions containing both boric acid and wheat germ oil have longer shelf life and color stability than other prior art ointment compositions containing mercuric oxide, the problem of discoloration still persists with these compositions. Improved means of stabilizing the yellow mercuric oxide and making it more resistant to discoloration are required.